You have two options to add an internal second drive. One is to install an mSATA SSD into the slot under the keyboard. The other is to buy a caddy for a HDD or SSD, and pop it in where the DVD is now. The caddy internally has a standard pinout, and will take any SATA drive, II or III.

I took the second option, and have an external DVD connected to my mini-dock. I have the internal DVD in my carry bag, but rarely use it.